The invention relates to oligomeric photosensitizers for use in cationic photopolymerizations.
Cationic polymerization is employed in many commercially important applications, including, for example, decorative and abrasion resistant coatings, printing inks, adhesives, fiber reinforced composites, microelectronic encapsulations, tan coatings, pressure sensitive adhesives, high performance aerospace composites, fiber optic coatings, stereolithography, photoresist and holographic recording media. The term xe2x80x9cUV curexe2x80x9d has also been applied to such processes because the polymerizations are typically induced by light having a wavelength in UV region below about 450 nm.
Cationically photopolymerizable, or photocurable, compositions typically contain one or more monomers or oligomers having epoxy or vital ether functionality, and a photoinitiator.
Currently, the most commonly used photoinitiators employed for photoinduced cationic ring-opening polymerizations are diaryliodonium salts, I, and triarylsulfonium salts, II, with the general structures shown below, in which MtXnxe2x88x92 represents a weakly nucleophilic counterion such as BF4xe2x88x92, SbF6xe2x88x92, PF6xe2x88x92, and (C6F5)4Bxe2x88x92. Another class of photoinitiators that show considerable promise is dialkyphenacylsulfonium salts, III. (See J. V. Crivello, J. H. W. Lam, Polymer Preprints 1979, 20, 415.) 
These three different types of photoinitiators possess high quantum yields of photolysis and are efficient photoinitiators of cationic polymerizations when irradiation is carried out using light in the short wavelength UV region (230-300 nm). A wide variety of vinyl and heterocyclic monomers undergo facile photoinduced cationic polymerizations using these photoinitiators.
Many of the above mentioned commercial applications of photopolymerizations are continuous, high speed web-base processes, and require correspondingly very high polymerization rates (cure speed). Therefore, there is a continuing need for highly efficient photoinitiator systems.
For maximum efficiency, the photoinitiator system must possess strongly absorbing chromophors that match as closely as possible the emission bands of the light source. The most intense emission bands of common light sources such are mercury arc lamps typically lie predominantly at wavelengths above 300 nm. However, most onium salt photoinitiators absorb most strongly at wavelengths below 250 nm.
One approach to increasing cure speed has been the use of photosensitizers to increase the response of photoinitiators to longer wavelengths. Electron-rich polynuclear aromatic compounds such as anthracene pyrene, perylene, coronene, 9,10-diphenylethynylanthracene, and carbazole compounds have been used with some success. However, these photosensitizers have serious drawbacks that limit their utility in UV cure applications. Anthracene and pyrene, for example, are volatile and can be lost during coating operations, with loss of effectiveness. Further, most polynuclear aromatic compounds (among them anthracene, pyrene, perylene, phenanthrene, and coronene) are acutely toxic, as well as potentially carcinogenic. Health and safety considerations, therefore, limit their use in many applications, especially in highly attractive food contact applications. As a result, there is a need for non-volatile and non-toxic photosensitizers.